Web tension control apparatus

ABSTRACT

A travelling web being wound onto or unwound from a reel has its tension sensed by a dancer roll in a web loop, and the position of the dancer roll is used to control the drive or brake system of the reel to maintain the tension uniform. With pneumatic control, a valve in the air line is actuated by the dancer roll to increase or decrease the supply, and in addition there is feedback from the air line to operate devices which restore the valve to its median position. These devices act in opposition, one acting faster than the other to cause the initial restoration movement. There is a resultant phase difference between dancer roll movement and valve actuation, which can be further modified by interposition of a mechanical phase shifting device between the dancer roll and valve.

G United States Patent [191 [1.11 3,782,653 Scott Jan. 1, 1974 WEBTENSION CONTROL APPARATUS 3,312,415 4/1967 Jan 43ml,-tml flii jInventor: Graham Robin Scott J 3,164,333 1/1965 Robertson 242/75.43Thomburx near England Primary ExaminerGeorge F. Mautz [73] Assignee:Masson Scott Thrissell Engineering Assistant Examiner-Edward J. McCarthyLimited, Bristol, Great Britain Attorney-Irvin S. Thompson et a'l.

[22] Filed: Mar. 20, 1972 [57] ABSTRACT A travelling web being woundonto or unwound from a reel has its tension sensed by a dancer roll in aweb 30 Foreign Application priority Data loop, and the position of thedancer roll is used to Mar. 20, l97l Great Britain 7,900 71 9"" thedFiVe of the F to main tam the tension uniform. With pneumatic control,a 52 us. (:1. 242/7543, 226/38 Yalve in the line is actuated by h 9? to[5 1] hm am B65}, 25/18, B65h 25/04 B65h 25/22 ncrease or decrease thesupply, and in add tion there [58 Field of Search 242/7543, 75.53;feedback from devlces b 226/38 44 restore the valve to its medianposition. These devices act in opposition, one acting faster than theother to [56] References Cited caltise tthehinitieggfstoratiogitrnovemgnt. Therell is a resu an p ase l erence e ween ancer ro move-UNITED STATES PATENTS ment and valve actuation, which can be furthermodigf 33-7 5 4 fled by interposition ofa mechanical phase shiftingdeemz.... llmgls 8/1963 Aaronm H 242/7143 vice between the dancer rolland valve. 3,057,574 10/1962 Justus 242/7543 16 Claims, 12 DrawingFigures PATENTED JAN 1 I974 SHEET 1 OF 6 Fla. 2.

WEB TENSION CONTROL APPARATUS The invention relates to pneumatic orhydraulic web tension control apparatus, for use in machinery, such aspaper reel unwinding machinery, in which a travelling web is tensionedby a web drive/brake system.

In such machinery it is common practice to use a sensing roll, known asa dancer roll, which is supported by a downward loop in the web.Increase in tension tends to shorten the loop and raise the dancer roll.A mechanical link to the dancer roll thus provides a position input fora tension control system, which may be loaded by weights, springs orfluid-operated actuating devices. The total effective mass of the dancerroll should be kept to a minimum to minimise inertia effects. The loadshould be substantially constant, e.g., as provided by a spring or verylow rate, and friction should also be kept to a minimum. Damping hasgenerally been required but is undesirable in that it adds a variableelement to the load. As is well known, the gain of the tension controlapparatus must be kept to a low figure in the range of frequenciesaround it natural periodic frequency of the system, in order to avoidinstability.

Pneumatic web tension control apparatus has previously been proposed inwhich the gain is reduced from a high value at zero frequency toa lowerfigure at middle and higher frequencies, by providing the control outputwith components which are proportional to the signal from the sensingroll and the integral thereof. The integral component provides aresetting action which is valuable when the required web tension is tobe adjusted.

According to the present invention there is provided a web tensioncontrol apparatus for a travelling web tensioned by a web drive/brakesystem, the tension control apparatus including a signal generatorarranged to produce an input signal in dependence upon the effective webtension, and transmission means arranged to receive said input signaland to supply a control output to the drive/brake system, saidtransmission means being adapted to introduce a forward phase shiftwhich is frequency-dependent over a middle band of frequencies. In somepreferred embodiments said transmission means includes a pneumatic ofhydraulic gain and phase-shaping network adapted to reduce the gain ofthe tension control apparatus in said middle band of frequencies whilethe forward phase shift increases with frequency.

Preferably this shaping network includes a pair of pneumatic orhydraulic actuating devices arranged in opposite senses and withdifferent time delays, the actuating device with the smaller time delaybeing arranged to act in the opposite sense to said input signal so asto protrude a component of the control output which is a derivativefunction of the input signal and thereby introduce the forward phaseshift.

The actuating device with the greater time delay, acting in the samesense as an input signal, provides an integral effect to reset thesignal generator to a median position. In a preferred form the actuatingdevices are connected in parallel to receive inputs from a pressurefluid line carrying the control output to the brake system, throughrestrictors of different throttling characteristics giving the differenttime delays. The outputs of these devices may be mechanically connected,in opposition to one another, to a control valve controlling thepressure of fluid in the line, the valve also being connected to receivethe input from the signal generator. This connection may be by means ofa beam actuated at one end by the signal generator, supported by theactuating device at the other end, with the control valve beingconnected thereto at an intermediate position.

In this form the actuating devices are preferably pneumatic bellows andthey may be sprung, or augmented by spring means, to give stiffness.

In a preferred alternative form, the input from the signal generator isarranged to cause relative movement between the two actuating devices,moving as one, and the control valve, these devices conveniently beingformed by diaphragms.

There may be two diaphragms which close off opposed spaces in a commonhousing and which are mechanically connected to a first operating memberof the control valve which is disposed between them. The input from thesignal generator is arranged to cause relative movement between thehousing and a second operating member of the control valve, one of theseoperating members being the valve body.

Othe embodiments using diaphragms have three in parallel linked by avalve spindle and mounted in a fixed housing to provide two closedspaces at opposite sides of the central diaphragm which communicate, viasaid restrictors with the pressure fluid line. The diaphragms/spindleassembly is normally balanced between spring means and the pressure offluid in said line, and said input signal is applied to the valvespindle.

In other preferred apparatus said transmission means includes amechanical phase shifting system comprising a combination of springmeans and viscous damping means arranged so that an input at one pointin the system results in a modified output at another point with lowgain at low input frequencies and gain increasing with frequency oversaid middle band of frequencies.

This system may be arranged so that the output phase lead achieves apeak value within said middle band of frequencies, and so that the gainapproaches unity at high frequencies.

To achieve this, the system may include an assembly of first springmeans and viscous damping means coupled in parallel and in series withsecond spring means reacting against a fixed structure, the input beingto the free end of said assembly and the output from a point effectivelyintermediate said assembly and said second spring means. In a preferredembodiment said viscous damping means is a rotary dashpot arranged to bepartially rotated by said input, the first: spring means acting betweenthe dashpot body and an output member and the second spring means actingbetween a fixed structure and saidmernber. The spring means areconveniently pairs of opposed springs, with the second spring meansstiff compared with the first spring means.

In an alternative arrangement, said system includes an assembly of firstviscous damping means, spring means and second viscous damping means toone end of which the input is applied and the other end of which reactsagainst a fixed structure, the output being from a point effectivelybetween the spring means and the viscous damping means at said one end.

The invention may be performed in various ways and some specificembodiments will now be described in more detail by way of example andwith reference to the accompanying drawings, in which:

FIG. 1 is a schematic elevation of a paper reel unwinding machineincorporating a web tension control apparatus,

FIGS. 2 to 8 are diagrams of pneumatic web tension control apparatusthat can be incorporated in the machine of FIG. 1,

FIG. 9 is a schematic diagram of control mechanism with phase advancingcharacteristics,

FIG. 10 is a graph illustrating frequency-gain and frequency-phase leadcharacteristics of the mechanism of FIG. 9,

FIG. 11 is a part-sectional elevation of a practical embodiment of themechanism of FIG. 9, and

FIG. 12 is a schematic diagram of an alternative phase advancemechanism.

Referring to FIG. 1, a web 10 is unwound from a reel 11 by means of apair of drawing rollers 12. The web 10 passes over idler rollers 13,14and 15 and around a dancer roll 16, forming a downward loop.

The dancer roll 16 is of comparatively light construction and is mountedbetween the free ends of a pair of pivoted arms 17, only one of which isshown, the other ends of which are pivoted at 18 to a fixed structure. Asubstantially constant downward force is applied to the dancer roll totension the web 10. This force may be applied by a spring of very lowrate or by a pneumatic or hydraulic actuator with low friction. Use ofweights to load the dancer roll 16 is also possible but is undesirablefrom the point of view of the inertia involved.

The arms 17 are connected by a mechanical link 19 to web tension controlapparatus indicated as a block 20 in FIG. 1 and which may take variousforms as shown in the remaining Figures. The control apparatus 20controls the pneumatic or hydraulic pressure in a line 21 to a brake 22acting on the reel 11, to adjust the tension in the web 10. The link 19provides a position input from the dancer roll 16. If the tension in theweb 10 increases, the effective length of the web between the idlerrollers 14 and 15 will decrease and the dancer roll 16 will rise, thusraising the link 19. Conversely a decrease in tension will cause a dropof the dancer roll 16 and link 19.

Referring now to the control apparatus shown in FIG. 2, the lower end ofthe link 19 is pivotally connected to one end of a substantiallyhorizontal beam 1, the other end of which is supported between a pair ofopposed bellows 2,3 which also act as compression springs, althoughseparate springs may be assembled within the bellows or adjacent them.The beam 1 is also connected near its mid-point to the spool 4 of acontrol valve 5, controlling the pneumatic pressure in the line 21 tothe brake 22. The central land 4a of the spool does not quite cover theoutlet port 5a of the valve 5 in the central spool position andconsequently the outlet port 5a is then open to both inlet port 51) andexhaust port 50 simultaneously. The pressure established at the outletport 5a, and hence in the line 21, varies in dependence on the bias ofthe spool 4 towards either supply or exhaust. This design can give avery high gain and a substantially linear pressure/displacementcharacteristic over a desired pressure range.

Branches 6,7 from the line 21 lead in parallel to the interiors of thepair of bellows 2,3, so that the bellows act on the beam in oppositionto one another. Restrictors 8 and 9, are incorporated in the branches6,7 respectively, restrictor 8 having a smaller throttling effect andthus providing a smaller time delay than restrictor 9.

When a position input is received from the dancer roll 16 via the link19, indicating a change in tension of the web 10, the first effect is toadjust the control valve 5 so as to apply or release the brake 22 toreadjust the tension. The change in pressure in the line 21 istransmitted with differing time delays through the restrictors 8 and 9to the bellows 2 and 3. The bellows 2, being connected to the restrictor8 giving the lower time delay, reacts more rapidly and adjusts the beam1 in the opposite sense to the input received through the link 19. Asthe pressure change is transmitted to the bellows 3, the pressures inthe two bellows gradually become equal and the beam is restored towardsits mid position. The effect of this is to give a control output to thevalve 5 which has one component which is proportional to the input, asecond component (produced by bellows 2) which is a derivative functionof the input and thereby introduces a forward phase shift whichincreases with frequency and reduces the gain of the tension controlapparatus in a middle band of frequencies, and a third component(produced by bellows 3) which provides an integral effect, resetting thebeam to its middle position after adjustment of the web tension.

With this arrangement, the gain at zero frequency is very high. The gainand phase shaping network formed by the bellows 2 and 3 and theirconnections reduces the gain in a middle range of frequencies to a valueat which instability is avoided, this range being arranged to be in theneighbourhood of the normal periodic frequency of the system, but thegain increases again as the frequency is increased, due to the forwardphase shift introduced by the bellows 3.

In the control apparatus of FIG. 3, the lower end of the link 19 isconnected to a housing 31 which contains a pair of diaphragms 32 and 33connected to opposite ends of a valve spool 34 and closing off opposedend chambers in the housing. Springs 35 and 36 within those chambers acton the diaphragms to provide the necessary stiffness. The valve spool 34is free to move axially within the valve body 37 which is fixed withrespect to the machine, indicated at 38. The valve is similar to that ofFIG. 2 and the position of the valve spool 34 within the valve body 37determines the pressure supplied to the line 21 as derived from theregulated supply pressure input to the valve at port 37b. The pressurein line 21 is applied to each diaphragm through branch lines 6 and 7equivalent to those of FIG. 2 and also incorporating respectiverestrictors 8 and 9, restrictor 8 having a smaller throttling effect andthus providing a smaller time delay than restrictor 9. The lines 6 and 7supplying the diaphragm chambers are necessarily of flexibleconstruction to enable relative movements between valve body 37 andcontroller housing 31 to occur.

The pressure in line 21 is also applied to the brake 22. As the controlapparatus is essentially a relatively small device, to maintain anadequate frequency response, it may not be capable of supplying thebrake 22 and associated lines with the necessary flow. To overcome thisa flow booster 39 may be incorporated in the brake line as shown, or apneumatic to hydraulic converter close to the control apparatus, thebrake then being hydraulically operated.

When a position input is received from the dancer roll 16 via the link19, indicating a potential change in tension of the web 10, the firsteffect is to move the spool 34 in unison with controller housing 31. Thevalve 34,37 is phased so that this action changes the pressure in theline 21 and hence the pressure applied to the brake 22, in theappropriate sense.

The change in pressure in the line 21 is transmitted with differing timedelays through the restrictors 8 and 9 to the diaphragms 32 and 33. Thediaphragm 32, being connected to the restrictor 8 giving the lower timedelay, reacts more rapidly and adjusts the spool 34 within the valvebody 37 against the springs 35,36 in the opposite sense to the originalinput received through the link 19. As the pressure change istransmitted to the diaphragm 33 the pressures acting on diaphragms 32and 33 gradually become equal and the valve spool 34 is restored towardsthe position it occupied immediately after the original input from thedancer roll 16.

The effect of this is to give a control output to the valve spool 34which has one component which is'proportional to the input, a secondcomponent (produced by diaphragm 32) which is a derivative function ofthe input and thereby introduces a forward phase shift which increaseswith frequency and reduces the gain of the tension control apparatus ina middle band of frequencies, and a third component (produced bydiaphragm 33) which provides an integral effect, resetting the dancerroll 16 to its middle position after adjustment of the brake.

It will be appreciated that the action described above could equallywell be achieved if the link 19 were connected to the valve body 37 andthe controller housing 31 were fixed.

In the control apparatus of FIG. 4 the movement of the link 19 isapplied to a spring 41 connected to one end of the spindle 42a of apoppet valve whose head 42b seats at the other end on a housing 43. Thehousing contains a main central diaphragm 44 and two subsidiarydiaphragms 45 and 46 which between them define chambers with which thebranch lines 6 and 7 are respectively in communication. These lines alsocontain restrictors 8 and 9 of respectively quick and slow response. Thespindle 42a passes centrally through the diaphragms which are clampedthereto. A further branch 47 leads from the main brake pressure line 21into a space between the head 42b of the poppet valve and the diaphragm46, the effective area of the underside of the head being slightlylarger than the effective area of the diaphragm 46. The output pressurefrom line 21 is thus applied to both in opposition and the slightinequality means that in the equilibrium position there is a smallleakage from the poppet valve. A bias spring 48 acts between a bracket49 fixed to the housing 43 and the spindle 42 and surrounds the spring41.

This control apparatus operates as follows. Assuming the dancer rollmoves downwards, indicating insufficient brake torque, an increaseddownward force is applied via the spring 41 to the spindle 42a, causingthe poppet valve to seat more closely and to increase the pressure inthebrake line 21, which previously had a continuous leakage via thevalve. The pressure in the brake line 21 increases by an amountproportional to the dancer roll displacement and this increase is fedvia the lines 6 and 7 and their respective restrictors 8 and 9 toopposite sides of the main central diaphragm 44. By virtue of the lesserrestriction in the line 6, there is initially a net upward force tendingto re-open the poppet valve and reduce the brake line pressure. After aninterval, the pressures on opposite sides of the main diaphragm 44equalize and the balance of the valve is restored.

Corresponding action takes place if the dancer roll moves upwards,tending to relax the spring 41. The spring 41 should not become slack,but this is prevented by the spring 48 which pre-loads it and alsoproduces an upward force that counteracts the pressures on the upperdiaphragm 46 from the branch 47.

Referring now to FIG. 5, this shows a modification of FIG. 4 and omitsthe lower half, which is identical. In this case, a branch 51 from thebrake line 21 terminates in an orifice with which the end of a spindle52 cooperates to control the leakage therefrom. The jet from the orificeexerts a downward force on the spindle. In this case, however, theupward movement of the spindle increases brake line pressure, and so therestrictor 9 is the one with the quick response and the restrictor 8 theone with the slow response.

In the control apparatus of FIG. 6, the lower half is again similar tothat of FIG. 4 and is not shown. The spindle 61 operates a peanut valve62 comprising two linked balls 63 and 64 on either side of a throat 65.The pressure supply is through a line 66 and in the intermediateposition there is leakage past the balls 64 and 63 to atmosphere while aportion of the supply is taken from the throat via duct 67 to the line21. The ball 64 on the supply side is smaller than the exhaust side ball63 and this asymmetry gives a net downward force on the spindleproportional to the brake line pressure and balanced by the compressionspring. The branch lines 6 and 7 to opposite sides of the main diaphragmhave restrictors 8 and 9, as before, the restrictor 9 being the one withthe quicker response.

As with FIG. 5, an upward movement of the spindle 61 is required toincrease the brake pressure and it does this by seating the ball 63against the lower end of the throat 65 to reduce the leakage andunseating the ball 64 to give freer passage to the pressure medium. Theincrease in pressure is fed more rapidly through the line 7 to urge themain diaphragm back towards the central position. The opposite movementof the spindle 71 releases brake pressure by unseating the ball 63further and seating the ball 64 more firmly.

In the apparatus shown in FIGS. 4, 5 and 6, the brake line pressurereacts directly on the controller spindle, thereby giving potentiallyrapid response from the valve, and so the brake line pressure re-adjustsvery rapidly to a force change due to dancer roll movement. They havethe characteristics of force balance devices whereby any force imbalancebetween the output, input and two feedback force generators serves toopen or close the valve and thus create a force balance. The spoolvalves shown in FIGS. 2 and 3 will not have such quick response sinceafter the valve has been displaced to a new setting a finite timeelapses before sufficient air has been passed into or out of the brakeline to reestablish the new pressure, and the larger the volume of thebrake line and brake actuator the longer the delay.

FIGS. 7 and 8 show modifications of FIGS. 2 and 3 designed to improvethis response.

In FIG. 7, which is in most respects similar to FIG. 2, the spool 4 isconnected to the beam 1 by a link 71 which has an extension jointed to ashaft 72 connected to one end of a bellows 73 in direct communicationwith the pressure line 21 via a port 74. A spring 75 opposes expansionof the bellows 73.

This arrangement produces an immediate feed back around the spool valvewhich improves its response. For example, should the dancer roll fall,the increased pressure applied to the line 21 is also practicallyinstantaneously applied to the bellows 73, which expand to raise thelink 71 and restore the spool towards its median position.

In the control apparatus of FIG. 8, which is in most respects similar tothat of FIG. 3, a secondary diaphragm 81 is introduced between thediaphragm 33 and the valve, and the space between the diaphragms 33 and81 is in communication via branch 82 with the brake line 21. Thediaphragm 81 has an effective area somewhat smaller than the diaphragm33 and there is therefore a net upwards force proportional to the brakeline pressure, which in conjunction with the springs 35 and 36 moves thespool upwards. This is the direction tending to reduce brake linepressure and there is therefore negative feedback around the spoolvalve.

The finite displacement necessary to cause a change in output pressurein the steady state, which would result in a finite change in the datumposition of the dancer roll for different brake torque settings, can beeliminated by designing the controller such that the diaphragm orbellows which provides the positive feedback is slightly larger ineffective area than the diaphragm or bellows that provide negativefeedback. These differences in area, in conjunction with the rate ofsprings, can be made just to match the finite gain of the valve so thatany change in brake pressure results in a spool movement, in the steadystate, which is just sufficient to give that change in pressure.

It would be possible to have hydraulic rather than pneumatic operationofthe control apparatus, although this would tend to increase thecomplexity and cost. It would be necessary to modify the above describedapparatus by including compressible elements in the feedback paths,preferably between the restrictors and diaphragms or bellows so that areasonable volume of fluid has to flow through the restrictors to changethe feedback pressures. These compressible elements may take the form ofadditional bellows free to expand and contract as the pressure changed.Also, it would be necessary to encase the apparatus to collect escapedfluid and return it to a tank.

With the abovedescribed tension control apparatus the movement of thedancer roll is transmitted directly to the valve which forms thecontroller in the pneumatic line. However, it is inevitable that thesupply pipe or pipes to the bralte or brakes will introduce some phaselag into the system, thereby degenerating the system response and itsstability. The longer these pipes are, the greater the lag. The lag dueto a length of pipe is basically a combination of the time delayinvolved for a pressure wave to propagate the pipe length and alsoresistance/capacitance effects which give increasing signal attenuationas the frequency increases. Careful selection of the bore of the pipecan minimise this delay or lag, but not eliminate it. With pipe lengthsof only a few feet the effect is negligible but when pipe runs of tensof feet are necessary it becomes progressively more pronounced.Hydraulic operation would reduce this lag problem, but as mentionedabove it is desirable to have compressible elements in a hydraulicsystem corresponding to those described, and these would onlyre-introduce lag.

Mechanism for introducing a compensating phase advance in the control isdiagrammatically illustrated in FIG. 9, where a spring 91 and a viscousdamping element or dashpot 92 are mechanically connected in parallel. Toone end 93 of this assembly 91,92 is applied the movement of a dancerroll and a further spring 94 acts between the other end of the assembly91,92 and a fixed structure 95. Output movement, which is applied to acontroller in the pneumatic brake supply line, is taken at 96 from thejunction between the spring 94 and the assembly 91,92. The directions ofmovement are indicated by arrows. The spring 94 is many times stifferthan the spring 91.

Depending on the relative values of the spring rates and dashpotforce/displacement constant the output displacement at 96 is related tothe input displacement at 93 depending on the speed, and therefore thefrequency, of the input signal. At a very low input speed or frequencythe dashpot 92 will present negligible resistance to relative motionbetween input and output and the ratio of output-input displacementswill be close to K lK, K where K, and K are respectively the rates ofthe springs 91 and 94. In these conditions the output motion will be inphase with the input motron.

As the input frequency is increased the dashpot 92 provides a forceopposing relative motion between input and output and the ratio ofinput-output displacements increases above the minimum K /K K Thisratio, or gain, is frequency dependent and its logarithm is proportionalto the logarithm of frequency, this being shown in the lower half ofFIG. 10 where the gain is expressed in dB.

Above a certain frequency the dashpot 92 behaves as a solid link incomparison with the spring 94 and the output approaches the input inmagnitude, i.e., the gain approaches unity or 0 dB. The lower half ofFIG. 2 shows these three stages as straight line asymptotes, and it canbe seen that for a limited frequency range, hereinafter referred to asthe operating frequency range, the gain increases at a rate of 6dB/octave or 20 dB/decade.

FIG. 10 also shows the phase relationship between the output and input.At low and high frequencies there is substantially no phase difference,but particularly over the operating frequency range where the gain isfrequency dependent there is a phase lead which achieves a peak value.This, and the frequency at which it occurs, are directly related to thevalues of the time constants T and T respectively equal to B/K, and B/KK where B is the dashpot constant. The low frequency gain is alsorelated to the time constants, having the numerical value T /T,. Thus ifthe spring rates were chosen to give K 9K the low frequency gain wouldbe 0.] (-20 dB) and the peak phase lead value would be 57. By suitablechoice of spring rates and dashpot constants the variable parts of thegain and phase characteristics can be shifted along the frequency axisto any desired range. For given spring rates, increasing the dashpotstiffness raises both time constants by the same factor and in effectmoves the characteristics to the left to a lower frequency range. Theoperating frequency range can be chosen to give the device the desiredeffect in compensating for the brake line lag.

Referring now to FIG. ill a practical embodiment of the mechanism ofFIG. 3 employs a rotary dashpot 101 whose hub 102 can be oscillated byan arm 103 fixed thereto and to which the movements of the dancer rollare applied via a rod 104. The dashpot body (not shown) is attached tothe hub and partially rotates therewith. The co-axial output shaft 1105of the dashpot has a lever 106 clamped thereto and which is normallycentered by a pair of springs 1107 reacting against the body 102 and byanother pair of springs H08 reacting against a fixed structure 109. Thesprings 107 have the lower rate and act at the smaller radius and areequivalent to the spring 91. The springs are in opposed pairs so thattheir preload forces are in balance and to make a compact design. Thefree end of the lever 106 operates, by means of an adjustable length rod110, a controller lllll in the pneumatic supply line to the brake. Therod 110 is adjustable to facilitate setting up the system. The springsand the dashpot may also be adjustable to tune to the frequency rangedesired.

An alternative arrangement is shown diagrammatically in FlG. 12 wherethe input from a dancer roll is applied at 121 to a dashpot 122, aspring 123 and a further dashpot 124 in series, the dashpot 124 actingagainst a fixed support 125. The output movement is taken at 126,between the dashpot 122 and the spring 123. Again, by choosingappropriate constants for the spring and the dashpot, characteristicssuited to the frequency range desired can be obained.

These mechanical phase advancing systems are preferably used inconjunction with the hydraulic or pneumatic apparatus described withreference to FIGS. l to 8. However, it is possible that they may be usedwith other controlling devices.

I claim:

1. A web tension control apparatus for a travelling web tensioned by afluid operated web drive/brake system. the tension control apparatusincluding a sensor co-operating with the web to produce a physical displacement in either of two opposite directions depend ing on increase ordecrease of tension either side of a norm, a fluid control valve whichgoverns the supply to said drive/brake system and which is responsive tosaid physical displacement to restore the tension towards said norm, andfeedback means to said valve from said supply downstream of the valve toresist the valve movement caused by said sensor and, after a delay, toassist said movement, wherein the improvement comprises introducingdelay means into the feedback that resists the valve movement, theassisting feedback being still further delayed, and providing aresilient connection between the sensor and the valve which issufficiently stiff to transmit said physical displacement for actuatingthe valve but which substantially eliminates a force reaction from thevalve back to the sensor.

2. A web tension control apparatus according to claim 1, wherein saidfeedback means includes a pair of fluid operated actuating devicesarranged to act in opposite senses on the valve and supplied throughrespective delay means for the supply to the drive/brake system. 1

3. A web tension control apparatus according to claim 2, wherein afloating lever is connected at one point to said sensor, at anotherpoint between said actuating devices and at a third point to the valve,the rei 10 silience of the connection between the sensor and the valvebeing provided by said actuating devices.

4. A web tension control apparatus according to claim 3, wherein a linkprovides: the connection between the lever and said valve and. meansresponsive to the outlet pressure of said valve give negative feedbackto the sensor through said link.

5. A web tension control apparatus according to claim 2, wherein saidactuating devices comprise chambers with diaphragms that are linked tomove together by a movable element of the valve, and wherein springmeans interconnect said diaphragms and said sensor.

6. A web tension control apparatus according to claim 5, wherein ahousing is spanned by said diaphragms, whereby two spaced fluid chambersare formed, and the valve element is disposed within the housing betweenthe diaphragms.

7. A web tension control apparatus according to claim 6, wherein thehousing is connected to move with the sensor and the spring means actbetween the inside of the housing and the diaphragms.

8. A web tension control apparatus according to claim 7, wherein a thirddiaphragm is provided to close off a space between itself and one of theother diaphragms, this space being in communication with the outlet ofthe valve to provide a negative feedback to the sensor.

9. A web tension control apparatus according to claim 6, wherein thereare three parallel diaphragms, a valve spindle comprising said valveelement linking said diaphragms, a fixed housing mounting saiddiaphragms and providing two closed spaces at opposite sides of thecentral diaphragm, separate passageways which communicate between eachof said two closed spaces and said pressure fluid line, restriction ofdifferent throttling characteristics disposed in said separatepassageways to give two different time delays, and spring means normallybalancing the diaphragm-spindle as sembly against the pressure of fluidin said line, said input signal being applied through said resilientconnection to said valve spindle.

10. A web tension control apparatus according to claim 9, wherein saidvalve is a poppet valve and the fluid pressure in said line is appliedto a chamber defined by an outer diaphragm, the housing, and the head ofthe poppet valve which, when balanced, permits leakage of the pressurefluid.

11. A web tension control apparatus according to claim 9, wherein thevalve is constituted by an end of said spindle co-operating with anorifice from which the fluid under pressure jets to an extent determinedby the spindle position.

12. A web tension control apparatus according to claim 9, wherein thevalve has two elements on said spindle at opposite ends of a throat, thefluid under pressure being normally directed past one said element andinto said line, which communicates with said throat, and also past saidother element as leakage.

13.. A web tension control apparatus according to claim 1, wherein saidresilient connection includes a mechanical phase shifting systemcomprising a combination of spring means and viscous damping meansarranged so that the input thereto results in a modified output atanother point with low gain at low input frequencies and gain increasingwith frequency over said middle band of frequencies, the systemincluding an assembly of first spring means and viscous damping meanscoupled in parallel, second spring means in series therewith, and afixed structure against which said second spring means react, the inputbeing to the free end of said assembly and the output from a pointeffectively intermediate said assembly and said second spring means.

14. A web tension control apparatus according to claim 12, wherein saidviscous damping means is a rotary dashpot arranged to be partiallyrotated by said input, the first spring means acting between the dashpotbody and an output member and the second spring means acting between afixed structure and said member.

15. A web tension control apparatus according to claim 13, wherein saidsecond spring means is stiff compared with said first spring means.

16. A web tension control apparatus according to claim 1, wherein saidresilient connection includes a mechanical phase shifting systemcomprising a combination of spring means and viscous damping meansarranged so that the input thereto results in a modified output atanother point with low gain at low input frequencies and gain increasingwith frequency over said middle band of frequencies, the systemincluding an assembly of first viscous damping means, spring means andsecond viscous damping means in series to one end of which assembly theinput is applied and the other end of which reacts against a fixedstrcture, the output being from a point effectively between the springmeans and the viscous damping means at said one end. l

1. A web tension control apparatus for a travelling web tensioned by afluid operated web drive/brake system, the tension control apparatusincluding a sensor co-operating with the web to produce a physicaldisplacement in either of two opposite directions depending on increaseor decrease of tension either side of a norm, a fluid control valvewhich governs the supply to said drive/brake system and which isresponsive to said physical displacement to restore the tension towardssaid norm, and feedback means to said valve from said supply downstreamof the valve to resist the valve movement caused by said sensor and,after a delay, to assist said movement, wherein the improvementcomprises introducing delay means into the feedback that resists thevalve movement, the assisting feedback being still further delayed, andproviding a resilient connectioN between the sensor and the valve whichis sufficiently stiff to transmit said physical displacement foractuating the valve but which substantially eliminates a force reactionfrom the valve back to the sensor.
 2. A web tension control apparatusaccording to claim 1, wherein said feedback means includes a pair offluid operated actuating devices arranged to act in opposite senses onthe valve and supplied through respective delay means for the supply tothe drive/brake system.
 3. A web tension control apparatus according toclaim 2, wherein a floating lever is connected at one point to saidsensor, at another point between said actuating devices and at a thirdpoint to the valve, the resilience of the connection between the sensorand the valve being provided by said actuating devices.
 4. A web tensioncontrol apparatus according to claim 3, wherein a link provides theconnection between the lever and said valve and means responsive to theoutlet pressure of said valve give negative feedback to the sensorthrough said link.
 5. A web tension control apparatus according to claim2, wherein said actuating devices comprise chambers with diaphragms thatare linked to move together by a movable element of the valve, andwherein spring means interconnect said diaphragms and said sensor.
 6. Aweb tension control apparatus according to claim 5, wherein a housing isspanned by said diaphragms, whereby two spaced fluid chambers areformed, and the valve element is disposed within the housing between thediaphragms.
 7. A web tension control apparatus according to claim 6,wherein the housing is connected to move with the sensor and the springmeans act between the inside of the housing and the diaphragms.
 8. A webtension control apparatus according to claim 7, wherein a thirddiaphragm is provided to close off a space between itself and one of theother diaphragms, this space being in communication with the outlet ofthe valve to provide a negative feedback to the sensor.
 9. A web tensioncontrol apparatus according to claim 6, wherein there are three paralleldiaphragms, a valve spindle comprising said valve element linking saiddiaphragms, a fixed housing mounting said diaphragms and providing twoclosed spaces at opposite sides of the central diaphragm, separatepassageways which communicate between each of said two closed spaces andsaid pressure fluid line, restriction of different throttlingcharacteristics disposed in said separate passageways to give twodifferent time delays, and spring means normally balancing thediaphragm-spindle assembly against the pressure of fluid in said line,said input signal being applied through said resilient connection tosaid valve spindle.
 10. A web tension control apparatus according toclaim 9, wherein said valve is a poppet valve and the fluid pressure insaid line is applied to a chamber defined by an outer diaphragm, thehousing, and the head of the poppet valve which, when balanced, permitsleakage of the pressure fluid.
 11. A web tension control apparatusaccording to claim 9, wherein the valve is constituted by an end of saidspindle co-operating with an orifice from which the fluid under pressurejets to an extent determined by the spindle position.
 12. A web tensioncontrol apparatus according to claim 9, wherein the valve has twoelements on said spindle at opposite ends of a throat, the fluid underpressure being normally directed past one said element and into saidline, which communicates with said throat, and also past said otherelement as leakage.
 13. A web tension control apparatus according toclaim 1, wherein said resilient connection includes a mechanical phaseshifting system comprising a combination of spring means and viscousdamping means arranged so that the input thereto results in a modifiedoutput at another point with low gain at low input frequencies and gainincreasing with frequency over said middle band of frequencies, thesystem including an assembly of first spring means and visCous dampingmeans coupled in parallel, second spring means in series therewith, anda fixed structure against which said second spring means react, theinput being to the free end of said assembly and the output from a pointeffectively intermediate said assembly and said second spring means. 14.A web tension control apparatus according to claim 12, wherein saidviscous damping means is a rotary dashpot arranged to be partiallyrotated by said input, the first spring means acting between the dashpotbody and an output member and the second spring means acting between afixed structure and said member.
 15. A web tension control apparatusaccording to claim 13, wherein said second spring means is stiffcompared with said first spring means.
 16. A web tension controlapparatus according to claim 1, wherein said resilient connectionincludes a mechanical phase shifting system comprising a combination ofspring means and viscous damping means arranged so that the inputthereto results in a modified output at another point with low gain atlow input frequencies and gain increasing with frequency over saidmiddle band of frequencies, the system including an assembly of firstviscous damping means, spring means and second viscous damping means inseries to one end of which assembly the input is applied and the otherend of which reacts against a fixed strcture, the output being from apoint effectively between the spring means and the viscous damping meansat said one end.